Catherine Montchamp"oreau,*9' Jean-Frangois Ferveurt and Micheline Jacques*. *CNRS UA 693 ... (reviewed in HOFFMANN, TURELLI and SIMMONS 1986;.
Copyright 0 1991 by the Genetics Society of America
Geographic Distribution and Inheritanceof Three Cytoplasmic Incompatibility Types inDrosophila simulans Catherine Montchamp"oreau,*9' Jean-Frangois Ferveurt and Micheline Jacques* *CNRS UA 693 Dynamique du Ginome et Evolution, Universiti Paris VI, 75252 Paris Cedex 05, France, and YLaboratoire de Biologie etGhitique Evolutives, CNRS, 91 198, Gifsur Yvette, France Manuscript received December 27, 1990 Accepted for publication June 1 1, 199 1 ABSTRACT Wolbachia-like microorganisms have been implicated in unidirectional cytoplasmic incompatibility between strains of Drosophila simulans. Reduced egg eclosion occurs when females from uninfected strains (type W) are crossed with males from infected strains (type R). Here we characterize a third incompatibility type (type S) which is also correlated with the presence of Wolbachia-like microorganisms. Despite the fact that the symbionts cannot be morphologically distinguished, we observed complete bidirectional incompatibility between R and S strains. This indicates that the determinants of incompatibility are different in the two infected types. S/W incompatibility is unidirectional and D. simulans strains showed that type S similar to R/W incompatibility. A worldwidesurveyof incompatibility was found only in insular populations which harbor the mitochondrial type SiI. Both WandR types were found among mainland and island populations harboringthe worldwide mitochondrial type SiII. Type S incompatibility could beinvolvedin the reinforcement of the geographical isolation of Si1 populations.
YTOPLASMIC incompatibilities have been described in several orders of insects, and in many cases have been found to be related to endosymbiosis TURELLI and SIMMONS 1986; (reviewed in HOFFMANN, BREEUWER and WERREN 1990). They have been extensively studied in the mosquito Culex pipiens, which exhibits multiple systems of unidirectional and bidirectional incompatibilities, maternally inherited, between or within populations (LAVEN 1957;reviewed in SUBBARAO 1982). These incompatibilities are correlated with the systematic presence of a rickettsial symbiont, Wolbachia pipientis, in the germinal cells of the mosquitoes (YEN and BARR1973). There are no naturally uninfected populations of C. pipiens. In D.simulans unidirectional cytoplasmic incompatibility was first reported between Californian strains (HOFFMANN, TURELLI and SIMMONS 1986). Incompatibility occurs when males from infected populations (type R) are crossedwithfemales from uninfected populations (type W). Mating and oviposition occur normally, but a large proportion of eggs failto hatch. The reciprocal cross is compatible as are crosses betweentype-R strains and betweentype-W strains. These twotypesof populations are widely spread 1988). W/ across the world (HOFFMANN and TURELLI R incompatibility is correlated in R strains with the presence of cytoplasmicmicroorganismswhich are morphologicallyindistiguishable from W. pipientis (LOUISand NICRO 1989; BINNINCTON and HOFFMANN
C
' To whom all correspondence should be addressed. Genetics 129: 399-407 (October, 1991)
1989). More recently, an Hawaiian strain of D.simulans containing microorganisms was found to be bidirectionally incompatible with a type R strain (O'NEILLand KARR 1990). This strain probably belongs to the new incompatibility type we report here and propose to call type S, since our reference strain comes from the Seychelles archipelago. MATERIALS AND METHODS
D. simulans strains: The strains used are listed in Table 1. Most of them originate from several wild-caught females; four of them are isofemale lines.Our reference strains were Seychelles-81 (S) and Nasr'allah(W).Riverside (R) and Watsonville (type W), which are used as reference strains in the R/W incompatibility system, were kindly provided by M. TURELLI. Incompatibility tests: Tests were performed at 25 '. For each mass cross, 30 4-day-old virgin females were allowed to mate for 24 hr with 40 2-day-old virgin males, ina bottle containing standard medium. Individual testswere performed with a single pair of fliesin a vial,in the same experimental conditions. In all cases, flies were transferred to oviposit on fresh medium consisting of agar (5%) and saccharose (0.5%) supplemented with vinegar, fresh yeast and animal charcoal powder. After 12-24 hr the adults were discarded. Unhatched eggs were counted at least 24 hr after removal of the adults. The percentage of unhatched eggs was used as a quantitative measure of incompatibility between strains or individuals. Crosses withmore than 80% unhatched eggs (Uh >80%) were considered as incompatible, and those with lessthan 20% unhatched eggs (Uh
